Electronic apparatuses have had small sizes and large densities, and accordingly, circuit boards have a large number of circuits and components mounted thereon at high densities.
FIGS. 7A to 7F are cross sectional views of a conventional circuit board 501 for illustrating a method of manufacturing the board 501.
FIG. 7A is a cross sectional view of an intermediate material 21 for manufacturing the circuit board. The intermediate material 21 includes a prepreg sheet 121 and organic films 22 bonded to both surfaces 121A and 121B of the prepreg sheet 121 by a laminating method with, for example, a heat roller. The prepreg sheet 121 is made mainly of glass fiber woven cloth and thermoset resin, such as epoxy resin, impregnated in the glass fiber woven cloth. The thermoset resin is maintained at B stage by, for example, a drying method.
Then, as shown in FIG. 7B, a though-hole 23 is provided in the intermediate material 21 by a processing technique, such as laser piecing. The intermediate material 21 is then placed on a porous sheet and sucked through the porous sheet. While the intermediate material 21 is sucked, the though-hole 23 is filled with a conductive paste 24 by, for example, printing. Then, the intermediate material 21 is removed from the porous sheet, and has the though-hole 23 filled with the conductive paste, as shown in FIG. 7C. The conductive paste 24 is prepared by mixing and milling conductive particles, thermoset resin, hardener, and solving agent.
Then, as shown in FIG. 7D, the organic film 22 is peeled off to provide the prepreg sheet 121 having the conductive paste 24 protruding from the surfaces 121A and 121B. Copper foils 25 are placed on the surfaces 121A and 121B of the prepreg sheet 121.
Then, while being sandwiched between metal plates, such as SUS plates, the prepreg sheet 121 are heated and pressed, thereby curing the intermediate material 21 so as to provide the cured board 122. The conductive paste 24 is compressed to provide a via-conductor 124 electrically connected with the copper foils 25, as shown in FIG. 7E.
The copper foils 25 are patterned by a photolithographic method to provide circuit wiring layers 26, thus providing a circuit board 501 shown in FIG. 7F. If required, the circuit wiring layers 26 and the cured board 122 may be coated with solder resists. Alternatively, the circuit wiring layers 26 may be subjected to surface processing, such as plating.
FIGS. 8A to 8D are cross sectional views of a conventional multi-layer circuit board 503 for illustrating a method of manufacturing board 503.
FIG. 8A illustrates a core board 31, the circuit board 501 manufactured by the processes shown in FIGS. 7A to 7F. The core board 31 includes a cured board 132, via-conductors 133, and circuit wiring layers 32 corresponding to the cured board 122, the via-conductors 124, and the circuit wiring layers 26 shown in FIG. 7F, respectively.
As shown in FIG. 8B, prepreg sheets 36 and a conductive paste 35 similar to the prepreg sheets 121 and the conductive paste 24 shown in FIG. 7D are positioned on both surfaces of the core board 31. Then, copper foils 34 are placed on the prepreg sheets 36, and heated and pressed. This operation allows the prepreg sheets 36 to be cured boards 136, thereby providing a cured laminated structure 502 shown in FIG. 8C.
Then, the copper foils 34 are patterned by a photolithographic method to provide circuit wiring layers 37, thereby providing the multi-layer circuit board 503. If required, solder resists may be provided on the circuit wiring layers 37 and the cured board 136. Alternatively, the circuit wiring layers 37 may be subjected to surface processing, such as plating.
The processes shown in FIGS. 8A to 8D may be repeated for manufacturing a further multi-layer circuit board by using the multi-layer circuit board 37 acting as a core board.
FIGS. 9A to 9C are cross sectional views of the intermediate material filled with the conductive paste shown in FIG. 7C for illustrating a method of the intermediate material.
Similar to the prepreg sheet 121 shown in FIG. 7C, organic films 41 are bonded to surfaces 43A and 43b of the prepreg sheet 43 having though-hole 44 provided therein, as shown in FIG. 9A. The prepreg sheet 43 bonded to the organic films 41 is sucked to a porous sheet 45 while the though-hole 44 is filled with a conductive paste 42 by the suction.
Then, as shown in FIGS. 9B and 9C, the porous sheet 45 is removed, and the organic films 41 are peeled off from the prepreg sheet 43. Then, the conductive paste 42 has portions 42A and 42B protruding from surfaces 43A and 43B of the prepreg sheet 43 by the thickness of the organic film 41.
In order to mount components at high density, distances between terminals of the components are reduced, and cause the diameters of the lands on the circuit wiring layers. This reduces the diameter of each via-conductor.
The high-density mounting decreases sizes of the circuit wiring layer and the via-conductors. The circuit boards 501 and 503 manufactured by the conventional method, upon being thinner, may have large resistances at the joining between the via-conductor and the circuit wiring layer. This will be explained in more detail below.
FIGS. 10A to 10C are cross sectional views of another intermediate material 153 filled with the conductive paste for illustrating a method of the board 153.
Similar to the prepreg sheet 43 shown in FIG. 9A, organic films 51A and 51B are bonded to surfaces 53A and 53B of a prepreg sheet 53 of the material 153, respectively, and though-hole 54 is provided in the prepreg sheet 53, as shown in FIG. 10A. The prepreg sheet 53 with the organic films 51A and 51B is sucked to a porous sheet 55 while the though-hole 54 is filled with the conductive paste 52 by the suction.
Then, as shown in FIG. 10B, the porous sheet 55 is removed. Then, the organic films 51A and 51B are peeled off from the prepreg sheet 53, as shown in FIG. 10C. This allows the conductive paste 52 to have a portion 52A protruding from the surface 53A of the prepreg sheet 53 by the thickness of the organic film 51A. However, a portion 56 of the conductive paste 52 may be separated when the organic film 51B is peeled off from the prepreg sheet 53. This causes the total amount of the conductive paste 52 to be smaller than the amount of the paste originally filling the though-hole 54. The resistance at the joining between the via-conductor and the copper foil on the circuit board accordingly increases, thereby causing connection fault.
FIGS. 11A to 11C are cross sectional views of a further intermediate material 163 filled with a conductive paste for illustrating a method of manufacturing the material 163.
Similar to the prepreg sheet 43 shown in FIG. 9A, organic films 61 are bonded to both surfaces of a prepreg sheet 63 of the material 163 having a though-hole 64 provided therein, as shown in FIG. 11A. The prepreg sheet 63 with the organic films 61 is sucked to a porous sheet 65, while the though-hole 64 is filled with a conductive paste 62 by the suction.
Then, as shown in FIG. 11B, a portion 66 of the conductive paste 62 may be separated from the though-hole 64 when the intermediate material 163 is removed from the porous sheet 65. In this case, the though-hole 64 is not filled with the conductive paste 62, preventing the circuit board from having a via-conductor to be connected the copper foil.